The present invention is in a method for producing a microporous, membrane-like polymeric film on a support material by forming a gel from an emulsion of a water and a polymer solution, and successively volatizing solvent and water after application to a sheet support. Such films are suitable for use as membranes for separating processes for the production and recovery of raw materials in the chemical, pharmaceutical, dairy and food industries. For example, supported membranes are used for the production of salt and drinking water (sea water desalting) by electrodialysis.
In the field of trauma therapy, for example, membranes of polyurethane or polytetrafluorethylene applied to a foam are used as a temporary skin replacement. The membrane keeps the wound moist, prevents the entry of bacteria, but allows oxygen necessary for the healing process to penetrate it.
Extremely moisture-repellent covers made of microporous films which are completely water-tight but nevertheless remain permeable to water vapor, are used on articles of clothing and in shoes, as well as in medical fabrics, operating-room covers and in clothing used in the area of an operation.
These films are manufactured chiefly by two different methods:
1. Formation of a polymer film by extrusion, by drying a polymer solution, or by polymerization in solution.
In the condensation or addition polymerization of polyurethane, for example, the starting components are dissolved in a solvent which can dissolve those components but not the end product. Thus, in Belgian Patent 719,272 the polyurethane becomes increasingly less soluble as additional polymerization progresses, until it finally precipitates.
In U.S. Pat. No. 4,466,931 a method is described which operates on a similar membrane-forming principle. A monomer or oligomer solution is photopolymerized, thereby spontaneously exceeding its solubility, and precipitates as a microporous film.
2. Formation of a gel layer by a sol-to-gel transition, following by drying.
The second principle includes all processes in which the polymer, after being applied, is coagulated out in a bath of precipitating agent or by applying a vapor of precipitating agent. It is necessary, then, that the precipitating agent be soluble in the polymer solvent.
The most widely used method is the coagulation of polyurethane, dissolved in dimethylformamide, by water. Disadvantages of that technique are that the coagulation process is slow and the washing out of the solvent and its regeneration are very complex.
Also state of the art is evaporative coagulation, which also involves the formation of a gel. The polymer solution consists of a good solvent and a poor solvent for the polymer. The poor solvent serves as a swelling or precipitating agent for the polymer. The good solvent must be more volatile than the precipitating agent. During the drying process the more volatile solvent evaporates first, and the swelling or precipitating agent becomes increasingly concentrated. Finally, after a critical concentration is reached, precipitation of the polymer occurs, i.e., the formation of the membrane structure. This process is described, for example, in DE-PS 16 94 059. A special case of such evaporative coagulation is the process described in Melliand Textilberichte 11 (1986), 824 to 829, where a hydrophobic polyurethane is dissolved in methyl ethyl ketone, some toluene is added as the swelling or precipitating agent, and water containing a hydrophilic polyurethane emulsifier is emulsified in this solution by means of a mixer or homogenizer.
Water is soluble 30 parts in 100 parts of methyl ethyl ketone of 20.degree. C. Thus not only the toluene but also the water act as diluents. A water-in-oil emulsion is formed. As the temperature is raised step-wise, first the methyl ethyl ketone is evaporated, then the toluene, and finally the water.
The above-listed processes have severe disadvantages. To assure a selective evaporation of the more volatile solvent, the evaporation must be performed at relatively low temperature, i.e., relatively slowly. The main reason for this is that the boiling points of the organic solvent, the organic nonsolvent ("poor" solvent) and of the water are relatively close. Another disadvantage is that the solvent and precipitating agent are soluble in one another to a degree, and separation for their recovery is very complicated and difficult, so that in practice they are simply disposed of by burning. Since very dilute solvent solutions are used, large amounts of solvent are involved, which increases the cost of the process for lack of an economical method of recovery.
It is therefore an object of the present invention to provide a method for the production of a microporous, membrane-like polymer film or coating which utilizes the formation of a gel from an emulsion of water and polymer solution, and in which the solvent and water are successively evaporated after application, either by a direct method or by the transfer method, to a support material.
Another object of the invention is to avoid the above-described disadvantages, especially the need for a large amount of solvent, the lengthy processing time, and the problems of disposal and recovery of the solvent as well as the use of precipitating or swelling agents.